Considerable quantities of aqueous hydrogen chloride solutions are obtained in the chemical industry. In particular, this is the case in the production of aromatic and aliphatic isocyanates. The recycling of chlorine from the solutions containing hydrogen chloride is usually carried out industrially by electrolysis of hydrogen chloride. To reduce energy costs, oxygen-consuming gas diffusion electrodes, for example, are used on the cathode side.
Such oxygen-consuming gas diffusion electrodes frequently use catalysts in order to reduce the cell potential necessary. These catalysts in many cases comprise noble metals, noble metal salts or noble metal compounds, for instance platinum or rhodium, so that the catalysts are generally very costly.
U.S. Pat. No. 6,149,782 discloses a catalyst comprising rhodium sulphide (RhSx) by means of which oxygen can be reduced. The catalyst is applied to a conductive web, if appropriate together with a binder, and thus forms an electrode which is suitable for the reduction of oxygen on application of an electric potential. Rhodium is a rare and thus expensive material, so that the same economic disadvantages as in the case of those based on other noble metals stand in the way of the use of the electrodes disclosed. A further disadvantage of the electrodes based on rhodium sulphide is their property on the cathode side of the selectivity for the reduction of oxygen decreasing at high current densities and hydrogen being formed as by-product. This limits the industrially achievable current density at which the reduction of oxygen at the electrode can still be operated reliably.
In U.S. Pat. No. 7,074,306 this disadvantage is alleviated by the addition of platinum to the rhodium sulphide. According to the disclosure, the accumulation of hydrogen in the O2 gas stream is prevented in this way even at high current densities. However, such an electrode is at least just as economically disadvantageous as that mentioned above, since the combination of rhodium and platinum further increases the price of the electrode.
U.S. 2006/0249380 discloses further suitable substances which can be employed as catalyst materials in the electrolysis of aqueous hydrogen chloride solutions. Apart from the abovementioned noble metals, rhodium and platinum, iridium, rhenium, ruthenium and palladium, their sulphides and oxides and also mixed phases, in particular with molybdenum and/or selenium, are disclosed as possible catalytically active materials. A material combination whose catalytic effect is not based on noble or transition metals is not disclosed.
The use of such noble metal catalysts also has the disadvantage that contact of the catalyst with chlorine and/or hydrochloric acid on the cathode side, too, cannot be reliably prevented during operation of electrodes in the electrolysis of hydrogen chloride and the materials mentioned react with chlorine and/or hydrochloric acid to form salts which can be leached from the electrode material. The performance of the electrodes can thus deteriorate as the period of operation increases and the life of the electrodes is limited due to consumption of catalyst material.
WO 2005/035841 discloses a process for producing nitrogen-doped carbon nanotubes on a conductive surface, in which the nitrogen-doped carbon nanotubes are deposited directly from a gas phase. This results in electrodes which can be used for the reduction of oxygen in batteries or fuel cells. The nitrogen-doped carbon nanotubes disclosed overcome the need to use expensive noble or transition metals as catalysts.
Only a limited variation in the layer thickness of the deposited material can be achieved in the direct deposition of the nitrogen-doped carbon nanotubes on the surface of the conductive material. A low layer thickness of the deposited nitrogen-doped carbon nanotubes has to be expected. In the electrolysis of hydrogen chloride, it is generally known to those skilled in the art that some leakage of the chlorine through the membrane from the anode side to the cathode side can frequently not be prevented. This chlorine is generally reduced again to chloride on the cathode side. Electrocatalytically active layers which are too thin, as are obtained according to the disclosure of WO 2005/035841, are disadvantageous for use in this technical field because corrosive attack on the material located under the catalytic layer has to be reckoned with. Furthermore, it is not possible to provide a sufficient number of active sites for the desired reaction in a thin catalyst layer, so that industrially feasible operation at high current density is not possible. Furthermore, WO 2005/035841 does not disclose any suitable arrangement together with a counterelectrode (anode) which would be able to be used in the electrolysis of hydrogen chloride.
It is therefore an object of the invention to provide an apparatus for the electrolysis of hydrogen chloride, which comprises an oxygen-consuming gas diffusion electrode which largely or completely dispenses with the use of expensive noble and/or transition metals and comprises catalytic materials which are not consumed or deactivated during operation and also has an increased selectivity for the reduction of oxygen at the electrode compared to known materials.